A commonly known capacitive micromachined ultrasonic transducer device is tens of micrometer size diaphragm-like cell comprising two electrodes opposing each other. For transmission the capacitive charge applied to the electrodes is modulated to vibrate/move the diaphragm (membrane) of the device and thereby transmit a sound wave. Objects placed in the way of the sound wave propagation reflect the wave. The reflected sound wave causes vibrations of the membrane, modulating the capacitance between two electrodes of the CMUT transducer, thereby generating an electric signal. This signal is representative for the reflected sound wave hitting the membrane.
One of the ways to drive a CMUT device is so called “collapsed mode” described in U.S. Pat. No. 8,203,912 B2. The DC actuation voltage applied across both electrodes is large enough deflect the membrane electrode towards the substrate electrode such that the membrane is brought into a physical contact with the substrate electrode and forms a collapsed area. An AC component applied between the electrodes is used to move the active area (not in contact) of the membrane. Schematically a CMUT device can be modeled as two capacitors in parallel. An active area capacitor Ca formed by an active area, wherein Ca value varies with the distance between the membrane electrode and the substrate electrode. A collapsed area capacitor Cc is formed by the collapsed area, and is constant for a given DC voltage. The latter capacitance is relative large, because of the small distance between the electrodes in the collapsed area. A large ratio between Cc and Ca has negative effect on an ultrasound array operating efficiency, wherein the array comprises these CMUT devices. In transmit mode, Cc forms an unwanted capacitive load to the driver, causing high currents to flow back and forth that do not contribute to acoustically emitted power. Practical implementations of transmitters may convert this power to heat, thus, resulting in overheating and efficiency loss. This may limit the possibilities to integrate transmitters onto the CMUT array. In receive mode, the acoustic power that is converted to electrical energy should go to the receiver. However, the large capacitor, Cc, in parallel to the variable capacitor acts as a low pass filter to this signal, reducing sensitivity of the device.
One of the ways to partially address these disadvantages is described in U.S. Pat. No. 8,203,912 B2. A three-terminal capacitive micromachined ultrasonic transducer device includes a substrate electrode split in two electrodes arranged in central and peripheral areas: a first electrode and a second electrode. The first and the second electrodes are arranged in laterally spaced relation within a common plane of the wafer. A membrane electrode located above the wafer includes a central region disposed in collapsibly spaced relation with the first electrode, and a peripheral region located outward of the central region and disposed in collapsibly spaced relation with the second electrode.
The first electrode and the second electrode are suggested to be electrically separated from each other, such that different bias voltages can be applied between the first and the membrane electrodes; and the second and the membrane electrodes. Though this solution permits isolating the parasitic capacitance of the collapsed central area, where the membrane is collapsed towards the first electrode; a practical realization of the three-terminal CMUT brings an additional complexity into providing interconnects between the CMUT device and the CMUT associate drive electronics.